I am a student at Hocking College in Nelsonville, Ohio. For our energy class, I am assigned the task of designing and building a sunspace for a mobile, energy-efficient building (16 feet wide × 16 feet long × 10 feet high). How do I correctly size this sunspace and make it usable as living space?
Kelley Barstow • via email
It sounds like a great project! The sizing of the sunspace depends on what your objectives are. The most ambitious goal would be to heat the cabin using only the sunspace, and even include solar-heat storage. You’d need to make sure that the cabin is well-insulated and tight. This is a very tall order.
A less ambitious goal is to keep the sunspace environment warm enough for plants all day, and warm enough for people during the day and for a few hours after sunset. Within these constraints, you’d seek to generate a bit of space heating for the cabin. This is the way a lot of conventional sunspaces are sized.
For example, let’s take a look at a less ambitious goal, and try to size a low-mass sunspace so that, on a sunny day, it generates heat for the cabin during the day and for a few hours after the sun sets. On cloudy days and for part of the night after a sunny day, the cabin would need another heat source. We’ll assume:
You’ll need to match the cabin’s heat loss to the sunspace’s heat gain. Looking at a sunny, 20°F day, the cabin’s heat loss (assuming the above construction details) comes out about 4,700 Btu per hour. (You can use the heat-loss calculator at bit.ly/BIS-HeatLossCalc.) Since you’re using efficient lighting and appliances, with a couple of people included, the internal heat gains might be 400 Btu per hour. That would give a net heat loss of 4,300 Btu per hour.
If the average outdoor temperature for the day is 20°F, then the 24-hour heat loss would be 103,200 Btu per day (4,300 Btu/hr. × 24 hrs.). This is about equivalent to 1.4 gallons of propane burned in an 80% efficient furnace.
That’s the simplified heat-loss calculation—you could be more accurate with a simulation that takes into account hourly temperature variations and other things. You could also do it for your actual weather for the whole heating season, day by day.
Our example sunspace solar heat gain looks like this: The solar insolation for a sunny winter day is about 300 Btu per square-foot•hour. If you assume that a LTMS can achieve 50% efficiency, then its solar heat output would be 19,500 Btu per hour (300 Btu/ft.2•hr. × 130 ft.2 × 0.5 efficiency).
This is more than four times the per-hour heat loss from the cabin. That’s good because:
You’ll also need to follow the other design rules that are covered in the article “Low Thermal Mass Sunspaces” in HP158 and at the link above. Of particular importance is summer venting—the sunspace will be an oven without it.
The simple building energy performance estimators that I use are easy to understand, but if you want to take it to the next level of simulation accuracy, you might want to check out eQUEST or EnergyPlus, which also offers a plug-in for Google SketchUp.
Both of these software packages are free, but both have fairly steep learning curves. Don’t get so wrapped up in these simulations that you lose sight of the basic physics that are going on.
Gary Reysa • builditsolar.com